1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus which can be preferably used as a light source for a projector apparatus.
2. Description of Related Art
A projecting-type projector apparatus is required to project an image being uniform and having sufficient color rendering properties on a rectangular screen, and hence a short-arc type discharge lamp having a mercury vapor pressure of at least, for example, 150 atmospheres at the time of lighting is employed as a light source.
Therefore, recently, a projector apparatus having a dimming function capable of adjusting brightness of the screen according to the brightness of an usage environment or the type of the image to be projected has developed, and as a discharge lamp lighting apparatus used in such a projector apparatus, the one including a power supply apparatus configured to light a discharge lamp by switching the mode between a rated power lighting mode in which rated power is supplied to the discharge lamp and a low power lighting mode in which power lower than the rated power, for example, 80% of the rated power is supplied to the discharge lamp is known (see Japanese Patent No. 4274053 and corresponding U.S. Pat. No. 7,436,121).
However, in the discharge lamp lighting apparatus having such a structure, there is a problem that flicker may occur in the discharge lamp when switching from the rated power lighting mode to the low power lighting mode.
The reasons why the flicker occurs in the discharge lamp in the low power lighting mode when the mode is switched to the low power lighting mode in this manner seem to be as follows.
When the discharge lamp is lit by the rated power lighting mode, as illustrated in FIG. 14(a), arcs A are formed between a pair of the electrodes 90, 91 of the discharge lamp, and hence evaporated electrode substance is accumulated by a halogen cycle at positions where original points of the arcs A at distal ends of the electrodes 90, 91 are positioned, so that projections p1 according to the magnitude of the original points of the arcs A are formed.
Then, in a mode switching term in which the mode is switched from the rated power lighting mode to the low power lighting mode, as illustrated in FIG. 14(b), the shape of the arcs A formed between the electrodes 90, 91 becomes narrower as the value of power supplied to the discharge lamp is lowered, so that the evaporated electrode substance is accumulated at positions on surfaces of the projections p1 of the respective electrodes 90, 91 where the original points of the arcs A are positioned, whereby minute projections p2 are formed on the surfaces of the projections p1 of the respective electrodes 90, 91 as illustrated in FIG. 14(c).
Therefore, since the shape of the arcs A formed between the electrodes 90, 91 is abruptly narrowed in the mode switching term in which the power supplied to the discharge lamp is lowered, the arcs A formed between the electrodes 90, 91 become unstable by the movement of the original points on the surfaces of the projections p1 of the respective electrodes 90, 91 as illustrated by broken lines in FIG. 14(d) and, consequently, a plurality of the minute projections p2 are formed on the surfaces of the projections p1 of the respective electrodes 90, 91 as illustrated in FIG. 14(e).
In this manner, in a case where the discharge lamp is lit in the low power lighting mode in a state in which a plurality of the minute projections p2 are formed at the distal ends of the electrodes 90, 91, the original points of the arcs A move between the plurality of minute projections p2 and, consequently, flicker occurs in the discharge lamp.
In the discharge lamp lighting apparatus of the related art having the light-dimming function, there is a problem that the distal ends of the respective electrodes are worn in the low power lighting mode, and hence the flicker occurs.
The reason why the wearing of the distal ends of the electrodes and the flicker occur in the discharge lamp in such a low power lighting mode seems to be as follows.
When the discharge lamp is lit, the electrode substance is evaporated from the electrodes by generation of high-temperature heat of the electrodes. However, the electrode substance evaporated by the halogen cycle accumulates on the surfaces of the electrodes. Then, when the discharge lamp is lit in the rated power lighting mode, evaporation of the electrode substance from the electrodes and accumulation of the evaporated electrode substance on the electrodes are balanced, so that wearing of the electrodes is suppressed.
Therefore, when the discharge lamp is it in the low power lighting mode, the amount of the evaporated electrode substance existing in the periphery of the electrodes is significantly reduced, and hence the accumulation of the electrode substance with respect to the electrodes is not sufficiently achieved. Therefore, evaporation of the electrode substance from the electrodes progresses, and consequently, the distal ends of the electrodes are worn, and hence the flicker occurs.